Manufacture of synthetic lubricants in the presence of phenothiazine



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Unite States Patent ()fitice Patented ,July 23, .1957

MANUFACTURE OF SYNTHETIC LUBRICANTS IN THE PRESENCE OF PHENOTI-IIAZINE No Drawing. Application October 30, 1953, Serial N 0. 389,478

4- Claims. (Cl. 260-485) This invention relates to synthetic ester manufacture and particularly to themanufacture of syntheticester lubricants. More particularly the invention relates to a method of preparingimproved synthetic .ester lubricants employing minor amounts of phenothiazine during the esterification reaction. Still more particularly, the invention relates to an improved method of preparing synthetic lubricants of the ester type which comprises the steps of esterifying acids with alcohols in the presence of from about 0.001% to 10.0%, preferably from about 0.05% to 2.0% by weight of phenothiazine which results in an improved yield of products having greatly improved properties of stability, "corrosivity, and load carrying ability.

In an effort to obtain superior lubricating oils having specific and unusual characteristics, new synthetic lubricants have been developed. One class which has attracted unusual interest as synthetic lubricants consists of those materials known to the art as esters. In general these synthetic lubricating oils are characterized by higher viscosity indices and lower pour'po intsthan mineral oils of corresponding viscosity. Lubricants possessing such properties are of special value in the lubricating of engines which are subjected to hightemperatures such as combustion turbine engines, particularly those of the prop-jet type. Mineral oil lubricants are generally less desirable for use in such engines because of'their tendency to result in a higher percentage of carbonaceous residue which accumulates on moving parts and interferes with the operation of the engine. In addition, the relatively high pour points and low flash points of such compositions are undesirable. The synthetic lubricants of the ester type are especially adaptable to use under such conditions since these lubricants have a desirable combination of high flash points, low pour points, and high viscosity indices, and eliminate, in a great number of applications, the necessity for additive agents.

In the manufacture of synthetic esterlubricants'to be used in blends for jet aircraft use, it is desiredthat the product have the following ranges of properties:

ASTMflash point, F 420-550 ASTM pour point, F Below 35 Viscosityat 210 .F., cs 5 to '11 Viscosityat 40.F., cs 3,000-25,000

'RCOOR' wherein R is the alkyl portion of the monobasic acid and wherein R is the alkyl radical of the alcohol used.

A monobasic acid may berea-cted with a polyhydric alcohol, one mole of the acid being used for each hydroxyl group of the'alcohol, for example:

2RCOOH+HO(CnH2nO)a:H

' RCOO(-CnH2nO)mOCR+2H2O wherein x is a number offrom 1 to about 20, and wherein n is a number of from 1 to about 6.

A monohydric 'alcoholmay be reacted with a dibasic acid to form a synthetic ester of the formula ROO.C(-CH2)zC0.0R wherein R is the alkyl radical of the alcohol and wherein x is a small whole number.

Complex esters may be prepared by reacting appropriate molar proportions of a 'monohydric alcohol, a

ipolyhydric alcohol and a dibasic acid. These complex esters-may be representedby the following five types:

TYPE I.MONOBASIC ACID-GLYCOL-(DIBASIC ACID-GLYCOL)-MONOBASIC ACID This complex ester may berepresented by the following structural formula:

TYPE II.ALCOHOL-DIBASIC ACID- GLYCOL- DIBASIC ACID x-ALCOHOL This material may be represented by the following formula wherein R1 and R5 are the combining radicals of the alcohol, R2 and R4 are the alkylene radicals of the dibasic acids, R3 is the alkylene radical of the glycol, and x is 0 to 5.

These esters are prepared in the manner similar to those of Type 1.

TYPE III.ALCOHOL-(DIBASIC ACID-GLYCOLh- MONOBASIC ACID These esters are prepared by reacting a dibasic acid and a glycol under such conditions that one hydroxyl group of the glycol-combines with-one carboxyl group of the dibasic acid, in other words, a half ester is formed. This half ester is then reacted with a molar proportion each of an aliphatic alcohol and a monobasic acid. These materials may be said to have the general formula wherein R1 is the combining radical of the aliphatic alcohol, R2 the alkylene radical of the dibasic acid, R the alkylene radical of the glycol, R4 the alkyl radical of the monobasic acid, and x is O'to 5.

The preparation of these ester materials is specifically set out in U. S. Patent No. 2,575,195.

TYPE IV.-ALCOHOL-'DIBASIC ACID-(GLYCOL- DIBASIC ACID)z-ALCOHOL These materials may be said to have the general formula :R1-:OOCR2COQQR3-OGCR4CQO)x-*R5 wherein R1 and R5 are the combining alkyl radicals of the alcohol, R2 and R4 the alkylene radicals of the dibasic acid, R3 is the alkylene radical of the glycol, and x is to 5.

It will be noted that the esters of Type IV have the same istructural formula as Type II. However, these complex esters are prepared by reacting an alcohol with a dibasic acid under such conditions that a half ester is formed and reacting two moles of such an ester with one mole of a glycol. The preparation of this type of synthetic ester lubricating oil is set out in detail in copending application, Serial No. 52,429 now U. S. Patent 2,703,811.

TYPE V.MONOBASIC ACID-GLYCOL-(DIBASIC ACID-GLYCOL)z-MONOBASIC ACID These synthetic esters may be said to have the general formula wherein R1 and R5 are the alkyl radicals of the monobasic acid, R2 and R4 are the alkylene radicals of the glycol, R3 is the alkylene radical of the dibasic acid, and x is 0 to 5.

It will be noted that these synthetic esters are the same as those appearing above under Type I except that this type is prepared by reacting a monobasic acid with a glycol under such conditions that a half ester is formed and reacting two moles of such ester with one mole of a dibasic acid. The details of the preparation of this type of synthetic ester are set out in U. S. Patent No. 2,575,196.

As was set out above, these ester materials are prepared by a combination of an alcohol, either monoor polyhydric or both, and an acid, either monoor dibasic, in various fashions.

Operable alcohols include the following:

2-ethylbutyl alcohol Z-butyloctyl alcohol Capryl alcohol Amyl alcohol Lorol alcohol The commonly known ether alcohol, formed by the reaction of ethylene oxide or propylene oxide with aliphatic alcohols and known in the industry as Dowanols, Carbitols, Cellosolves, etc. may also be used.

A group of alcohols especially adapted for use in connection with the present invention are the so-called Oxo" alcohols, prepared by the reaction of carbon monoxide and hydrogen upon the olefins obtainable from petroleum products. Materials such as diisobutylene and C1 olefins are suitable for this purpose; also higher and lower molecular weight olefinic materials are sometimes employed. The alcohols obtained in this manner normally have a branched chain structure.

Among the monobasic acids which may be employed in the preparation of the esters of the present invention, the following may be listed as illustrative:

Propiom'c acid Butyric acid Valeric acid Caproic acid Capric acid Caprylic acid Lauric acid Palmitic acid Any of the corresponding thio acids 2-ethylbutyric acid 2-ethylhexoic acid 0x0 acids, i. e. C8, C10, C16, etc.

Acids produced by oxidation of petroleum fractions Acids produced by alkali fusion of alcohols Stearic acid Illustrative examples of the dibasic acids which may be employed in the synthesis of the esters of the present invention are the following:

Succinic acid Glutaric acid Adipic acid Pimelic acid Suberic acid Azelaic acid Sebacic acid The polyhydric alcohols employed in preparing the esters of the present invention include ethylene glycol and any of the paraffinic homologues of the same containing up to 18 carbon atoms. These may include, for example, ethylene glycol, propylene glycol, butylene glycols, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, and the like. Since the glycols may also contain oxygen of sulfur atoms, compounds such as diethylene glycol, triethylene glycol, the polyethylene glycols of the formula HO (CHzCHzO nCHZCHZOH wherein n is l to 26, and the polypropylene glycols of the general formula I|R1 Illa R1 '2 HO(OHCHO)HOHCHOH where either R1 or R2 is a methyl group and the other is hydrogen, and where n is l to 20, may likewise be employed. Glycols containing sulfur atoms in thioether linkages may also be employed, and these include compounds as thiodiglycol and 1,2-bis(2-hydroxy-ethylmercapto) ethane. There also may be used glycols containing both oxygen and sulfur in similar linkages; such a compound is bis-2(2-hydroxyethoxy) ethyl sulfide.

As was described above, the complex ester synthetic lubricants may be prepared by reacting two of the ingredients to get the half ester. It is proposed herein that this procedure be followed by in addition a small amount, .001% to 10.0%, preferably about 0.05% to 2.0% by weight based on the weight of the finished ester, of phenothiazine oxidation inhibitor be added during the esterification. It is also proposed that beside the two-step procedures, a one-step procedure employing phenothiazine may be used.

The synthetic lubricants of the ester type are prepared preferably in the presence of a suitable esterification catalyst such as toluene sulfonic acid, sulfosalicylic acid, sodium acid sulfate, boric acid, phosphoric acid, zinc sulfate, etc. Although any of the conventional methods of esterification may be employed in making a complex ester, the one-step procedure is preferred primarily because of reduced charging and esterification times. The effect on quality and composition of the complex ester is not altered by the method of preparation providing the crude ester is subjected to the same general thermal treatment. In the one-step procedure all of the desired ingredients including phenothiazine, excess alcohol (or monoacid), water entrainer, solvent, and catalyst are comixed and heated simultaneously to about -220' C. until at least the theoretical yield of water or a low acid number is obtained. The esterification is then continued preferably at about 230 to 260 C. for several hours during which time the solvent, water entrainer and excess reactant are removed usually under a reduced pressure. The small amount of acidic compounds remaining are then removed (by treatment with alkali, alkali carbonate, etc.) to give the complex ester lubricant of this invention.

In the preparation of the complex esters using a twostep procedure, the dibasic acid and the polyhydric alcohol, usually a glycol, are admixed with the catalyst in the presence of a water entrainer. The mixture is heated to about 180 -220 C. for about 4-5 hours and then the monohydric alcohol is added in amounts that are in excess of theoretical. The total mixture is then heated to about 220 C. until such time that the acidity is at the (1) Maximum oxidation inhibition during all stages of "6 by the two-step procedures as shown also in the table. It will be noted that in only the two non-catalyzed systems the length of esterification as shown by Hours to 100% H2O was unduly long whereas when a catalyst was present the time required was normal. In the noncatalyzed runs the esterification was less complete as indicated'by the higher neutralization and 'hydroxyl numbers. The presence of a catalyst is therefore most desirable to insure maximum utilization of the glycol ingredients.

Table Ir -Complex ester preparation in presence of phenothiazine excess 2-ethy1hexanol; N aHSOi catalyst.)

-Moles sebacic acid Moles polyethylene 1Ycol of 200'mo1ecu1ar weight Esterlflcation Step W t fggtal' Maxl- 1 fNezit. gydzoryl E Este Pre oration Percent a er me, mum 0. mg. 0. mg x r p Entrainer Hours to Total Max. Hrs. Temp, KOH/ KOH/ 100% Time, Temp, C. gm.) gm.) H O Hrs. O.

No Catalyst 9% Toluene 12 '12 220 2 230 5. 4 19. O 0 26% Catalyst do 25 6 190 3 232 2. 3 5.0 0.36% Catalyst 4% Heptane... 1:5 3 210 3 242 0. 95 1.0 No Catalyst 0.5% Pheno- 9% Toluene. 12 1 2 220 2 225 5.1 18 0.26% Catalyst 0.5% Phen d0 2; 5 2. 5 190 5 230 2.3 5.0 0.36% Catalyst 0.5% Pheno 4%Heptane... 1. 5 3 210 3 240 O. 64 1.3 0.-'i+d7iqait)alyst 0.5%Pheno. (Z-Step'vla Glycol 6% Heptane... 3 4 210 3 249 0.64 4. 5

c1 v v 8 0.36% Catalyst 0.5% Pheri'o. (2 Ste'p via. 6% Heptane... 3 4 200 3 249 O. 63 5. 3

Alcohol Acid).

1 Main stripping step.

the preparation of the ester. Phenothiazine appears to be one of the best, if'not'the best, oxidation inhibitor for synthetic lubricants, as far as is known.

(2) Decreased degradation at high temperatures.

(3) Lower oxidation-corrosion weight changes.

(4) Lower lead corrosion.

(5) Slightly higher flash points.

(6) Higher load carrying abilities.

(7) Increased product yields.

(8) Elimination of subsequent phenothia'zine addition and heating during blending operations.

(9) Possible elimination of deleterious "active agents which as insoluble phenothiazine-active agent complexes should be removed during washing and filtration steps. These precursors thereby never ,get a chance to cause trouble in an engine.

(10) Elimination of acidity build-up during storage.

Phenothiazine is essentially a neutral compound by virtue of its amino hydrogen being almost non-basic. Unlike most amine type oxidation inhibitors, it does not react readily with acid catalysts and hence does not cause a decrease in the rate of esterification. It is also essentially unreactive toward acidic ester components'or ingredients. It is reasonably soluble in synthetic esters but highly insoluble in alkali andaqueous solutions. Consequently unlike phenolictype oxidation inhibitors, it is not lost or removed during the washing stages in the purification of the ester. As far as is known phenothiazine introduces no deleterious effect other than the usual darkening of the ester lubricant and possibly a slight but expected increasing of the'low temperature viscosity.

A series of complex esters of the type Z-ethylhex-anolsebacic acid-polyethylene glycol-sebacic acid-Z-ethylhexanol have been prepared in which'the catalyst concentration and the addition of phenothiazinewere the main variables. The thermal conditions of preparation as shown in Table I were essentially the sameinall cases. For comparison the same complex "esters were prepared The materials prepared as shown in Table 1, Examples 1-8, were submitted to a series of standard tests to determine their lubricating and stability characteristics.

The viscosities of the products at 210 F. and at 40 F. -Were determined as were their flash and pour points. "The'neutralization number and the hydroxyl values were obtained. The lubricants were submitted to a lead corrosion test in which a weighed lead strip of about 2% by /2" is'twirled in a 500 cc. sample of the oil for one hour at 325 F. The leadstrip is then washed, reweighed and the loss inweight calculated.

The lubricants were also submitted to the Corrosion- Oxidation Stability Test. This test, described in detail in "Military Specification MIL-L-7-808-A, consists briefly in 'submerging five metal plates of known area and weight in a bath of gm. of the sample of the oil under test. The sample is then'raised'to a temperature of 347 F. and oxygen is bubbled through the bath. At the end of 72 hours of test time, the metal plates are removed and the weight change per square centimeter is calculated. The amount of weight change is directly proportional to the corrosivity of the breakdown products, that is, the :less corrosive are the breakdown products, the less the change of weight of the metal sample plates. The neutralizationnumber and the increase in viscosity at 100 F. of the sample is determined at the conclusion of the test and indicates the stability of the sample.

The complex ester lubricants of Examples 1 to 8 were also submitted to a load carrying test in the standard SAE machine. In this test the bearings were submerged in the oil and a 50 pound load was placed on the machine. Every 10 see. an additional 50 pounds was added to the load until the bearings failed. Two runs Were made on each oil and the average of the failure load was reported.

It is to be seen from the data reported in Table II below that the utilization of minor amounts of phenothiazine in accordance with this invention results in improvement in flash point, lead corrosion resistance, overallcorrosion-oxidation stability, load carrying properties, and yield.

Table II.-Prpertres of complex esters Viscosity, Os. Lead Product F. Pour Flash Corr. BAE- Yield, Ex. Ester Preparation Point, Point, Loss, Load, Percent F. F. Mg. Lbs. oi 210 -40 Theory N 0 Catalyst. 8. 50 15, 410 70 475 75 725 87. 9 0.26% Catalyst 10. 7 20, 900 65 '475, 70 700 96. 6 0.30% Catalyst- 10. 3 21,120 65 485 23 700 97. 7 N0 0atalyst+0.5% Pheno 8. 36 15, 450 75 495 98 775 93. 0.26% Catalyst-+0 5% Pheno 10.3 24, 820 -65 505 30 750 96. 8 0.36% Catalyst+0.5% Pheno 10.2 21, 380 -65 490 775 99.0 0.36% Catalyst+0.5% Pheuo (2Step 9.95 20, 570 -65 495 11 725 99.0

via Glycol+Aei 0.36% Catalyst-$0.57 Pheno. .(2-Step 9.87 20, 640 -75 490 750 97.6

via Alcohol-l-Acid).

Oxidation-Corrosion Stability 347 F.

Neut. Hy- Ex Ester Preparation N 0., droxyl MgJOm. Wt. Change inal Avis. me. No., Neut. [100 F. KOH mg. gm. KOH/ Cu Mg Fe Al Ag gm.

1. No Catalyst 27 -,15 o3 03 05 1. 4 10 0.13 12 43 32 55 92 61 1. 2 8 0.15 2. 0 yst 37 +1.1 a9 63 33 0 6 0.06 3.1 4 No Catalyst 0.5+ Pheno. +3. 4 0 01 0 4 5 21 14 5 0.26% Catalyst+O.5% Pheno .21 18 11 +.22 +.0 6 .0 6 0.30% Catalyst 0.5% Pheno- 10 53 22 23 30 7 3 17 3 7. 0.36% Catalyst 0.5% Pheno.

(Z-Step via Glycol Acid) 05 20 10 26 16 9 4 0- 1 4.0 S 0.36% Catalyst 0.5% Pheuo.

(2-Stcp via Alcohol+ Acid)" -.04 11 09 03 05 0.8 4 0. 21 6.6

EXAMPLE 9 30 Part BPhen0thzazme addltzon before str1pping.-The

In a round bottomed 2-liter flask fitted with a takeofi tube and a water-cooled following:

180 g. of butanediol-l,4 520 g. of C8 Oxo alcohol 584 g. of adipic acid 6.4 g. of phenothiazine 200 g. of xylene Neut. no 2.38 Vis./210 F. (cs) 7.56 Vis./100 F. (cs) 39.10

Corrosion-oxidation Stability Test (347 F./ 72 hours): mg./cm. wt. loss Copper 0.33 Magnesium 1.76 Aluminum 0.02 Iron 0.02

EXAMPLE 10 Part ANo phenothiazine addition.-The components of the esterification were as follows: 3,000 parts sebacic acid 1,350 parts polyethylene glycol 200 175 parts NaHSO4-H2O 400 parts toluene The mixture was heated 3 hours to a maximum of 380 F. To the product was added:

2,450 parts of 2-ethylhexyl alcohol' The solution was then heated for 6 hours toa maximum temperature of 430 F. v

The product was stripped for 6 hours at '450" F. and 30 mm. of mercury, washed with 10% soda ash solution, using isopropyl alcohol and heptane as emulsion breakers. The latter solvents were then stripped off at 2302- 250: F. and 30-50 mm. Hg. 7

condenser there was placed the constituents and reaction conditions were identical with Part A above except that prior to stripping 30 parts of phenothiazine were added to the ester.

Inspections on the products are listed below:

Part A Part B 10. 7 10. 5 55. 7 54. 9 55 485 490 Neutralization No. 0. 53 0. 17 Lead Corrosion, Mg. Loss at 325 F 231 67 To the esters produced in accordance with the instant invention other additive materials such as silicones, phosphites, phosphates, amines, sulfonates, rust inhibitors, viscosity index improvers, pour point depressants, load carrying agents, oxidation inhibitors, and the like, may be added to improve these characteristics. The esters may be used alone as lubricants, or they may be blended with other lubricating oils, either natural mineral oils or other synthetic lubricants. The esters, or blends containing them, may be used to formulate grease compositions by thickening with the common grease forming soaps.

In addition to the simple esters and the complex esters that are set out in detail above, it is also within the concept of this invention to treat modified simple esters, such as for example, those prepared in the presence of from 0.1% to 15.0% by weight, based on the weight of the reactants, of a selected group of compounds, particularly glycols and dibasic acids. For instance, the following examples set out in detail the preparation of a series of dibasic acid esters which are modified by the presence of a minor amount of a glycol.

MP E IL-PREPARATION OF DI-Z-ETHYLHEXYL SlBA ATE IN THE PRESENCE OF 2.9 WT. PERCENT OF POLYETHYLENE GLYCOL, 200 M. WT.--NO CATALYST Heptane Moles sebaeic acid :135

Ratio Moles glycol All of the above ingredients were comixed and heated in the absence of a catalyst to a reflux temperature of 156 C. where the water of esterification began to collect in the water trap. After 1 hour the liquid temperature was 222 C. and 100 cc. of water had collected. During the subsequent 3 hours the temperature gradually rose to 234 C. and a total of 132 cc. of water (99.2% of theory) had collected. The crude liquid esterification product was then stripped over the next hour at a maximum temperature of 236 C. at a pressure of 10 mm. of mercury. The crude ester product after this heat treatment was washed with 10% NazCOa solution and isopropyl alcohol followed by a wash with water and heptane and finally a wash with water and isopropanol. The modified di-2- ethylhexyl sebacate thus obtained had the properties shown in the table below.

EXAMPLE 12.DI-2-ETHYLHEXYL SEBACATE PRE- PARED IN THE PRESENCE 2.14% POLYETHYL- ENE GLYCOL AND NaHSOs CATALYST The following ingredients were used.

Sebaeie Acid 2-Ethylhexanol 8.5% Excess 2etl1yll1exan0l.

Moles sebacic acid Moles glycol All of the above ingredients were heated simultaneously in a one-step operation to a temperature of 230 C. for 3 hours during which time 136 cc. (102.8% of theory) of Water were collected. After heating this crude ester at 225 232 C. for an additional 3 hours at mm. pressure, the material was cooled and then decanted from the settled catalyst. The crude ester was then washed as described in Example 11. The physical properties of this lubricant are given in the table below. The corresponding properties for di-2-ethylhexyl sebacate are also given for comparison. It will be noted that the addition of a small amount of glycol in the preparation of Examples 11 and 12 markedly improves the viscosity-temperature (higher V.I.s) and flash properties of the diester. The most favorable improvement is in its increased load carrying ability. The viscosity remained about the same. These same improvements are not obtained by simply adding the same glycol to the diester because the glycol is largely incompatible with di-Z-ethylhexyl sebacate. The high lead corrosion observed with Example 11 indicates incomplete esterification and points out the advantage of a catalyst in esterification reactions.

Ratio -18.4

Table III Dl-2-Ethyl- Ex. 11 Ex. 12 Di-2- hexyl Ethyl- Sebacate-lhexyl 2.1% Sebacate PEG,200

Viscosity, Os. R:

210 3. 81 3. 82 3. 89 100 15. 3 16. 1 12. 8 0. 241 247 188. 6 40 1, 976 1, 956 1, 482 The PEG Viscosity Index 167 168 162 200 sepa- Pour Point, F -75 75 75 rates on Flash Point, F 455 465 430 standing Fire Point, F. 505 505 500 incorn- N out. No 0.11 0.08 0.1 patible. Hydroxyl N0 3.3 0.50 4.1 1 Hr. Lead Corrosion 7 18 SAE Test, Lbs. Load 625 425 1 All samples contain 0.5% Phenothiazine for the determination.

EXAMPLE 13DI-2-ETHYLHEXYL SEBACATE PREPA- RATION IN THE PRESENCE OF 21% POLYETHYLENE T200 AND LIME (CALCIUM SEBACATE) The following components were comixed and heated in a 1-step esterification procedure.

After heating to reflux temperature (153 C.), the temperature gradually rose to 250 C. over a 4% hour period. A total of only 134 cc. (93% of theory) of water was obtained. The material was then heated at 240- 245 C. while the pressure was lowered to 10-15 mm. After 3 hours the crude ester was allowed to cool before filtering off the white catalyst. No carbonization occurred and a light colored, clear lubricant was obtained.

To summarize briefly, the instant invention relates to an improved process for the formation of synthetic lubricating compositions of the ester type. Particularly the invention teaches the art an improved process which comprises esterifying alcohols with acids in the presence of minor amounts of phenothiazine to improve yields, viscosity properties, product stability, corrosivity, and load carrying ability.

What is claimed is:

1. In a process for the preparation of complex ester synthetic lubricants from branched chain alcohols, glycols, and dicarboxylic acid in the presence of an esterification catalyst, the improvement which comprises carrying out the esterification reaction in the presence of from about 0.05% to about 2.0% by weight of phenothiazine.

2. In a process for the preparation of complex ester synthetic lubricants having outstanding lubricating properties which comprises reacting a dicarboxylic acid and a glycol in the presence of a catalyst and a water entrainer at a temperature of about 180 to 220 C. for a period of time to remove substantially all of the water of esterification, subsequently reacting with the product so formed sufiicient monohydric alcohol to completely esterify the unreacted carboxyl groups of said dicarboxylic acid at a temperature of about 180 to 220 C. for a period of time suflicient to remove substantially all the water of esterification, and stripping the resulting product at reduced pressure, the improvement which comprises adding to the reaction mixture of dicarboxylic acid and glycol from 0.05% to 2.0% by weight, based on the weight of the final product, of phenothiazine.

3. In a process for the preparation of complex ester synthetic lubricants having outstanding lubricating properties which comprises admixing a glycol, a dicarboxylic acid and an alcohol in a molar ratio of 1:2:2 respectively with a water entrainer and an esterification catalyst, heating the mixture to a temperature of about 180 to 220 C. for a period of time to remove substantially all of the water of esterification, and stripping the product at a temperature of about 230 to 260 C. at a reduced pressure to remove the water entrainer and excess reactants, the improvement which comprises adding to the reaction mixture from 0.05 to 2.0% by weight, based on the weight of the ester, of phenothiazine.

4. A process according to claim 3 wherein said dicarboxylic acid is sebacic acid, said glycol is a polyalkylene glycol having a molecular weight of about 200, and said alcohol is 2-ethylhexanol.

References Cited in the file of this patent UNITED STATES PATENTS Saunders et a1 Sept. 2, 1952 

1. IN A PROCESS FOR THE PREPARATION OF COMPLEX ESTER SYNTHETIC LUBRICANTS FROM BRANCHED CHAIN ALCOHOLS, GLYCOLS, AND DICARBOXYLIC ACID IN THE PRESENCE OF AN ESTERIFICATION CATALYST, THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE ESTERFICATION REACTION IN THE PRESENCE OF FROM ABOUT 0.05% TO ABOUT 2.0% BY WEIGHT OF PHENOTHIAZINE. 